|
|
| Re: Is there an automated way to estimated FWHM on 2-D image [message #68075 is a reply to message #68026] |
Sun, 20 September 2009 19:41  |
Jeremy Bailin
Messages: 618
Registered: April 2008
|
Senior Member |
|
|
On Sep 20, 9:12 am, wlandsman <wlands...@gmail.com> wrote:
> On Sep 20, 8:10 am, Jeremy Bailin <astroco...@gmail.com> wrote:
>
>> On Sep 19, 9:02 pm, John Shaw <jds...@udel.edu> wrote:
>
>>> I was wondering if anyone had a routine for estimating the full-width-
>>> at-half-maximum (FWHM) of possible point sources in a 2-D array. Most
>>> of the routines I have found and examined request the FWHM for a
>>> guassian to be convolved to find the sources.
>
>> Would PKFIT in the IDL astronomy library give you what you need? You
>> can get the Gaussian dispersion out, which is easy to convert into a
>> FWHM.
>
> I would just use any Gaussian-2d fitting routine, such as
> gauss2dfit.pro in the ITTVIS library, or (preferably) the Gaussian
> option of the mp2dfitfun.pro function in Craig Markwardt's fitting
> library (http://www.physics.wisc.edu/~craigm/idl/fitting.html).
>
> One thing to be careful of is the choice of the fitting region
> size. We don't observe Gaussians in real life, and for example,
> star images have very extended wings. If your fitting region
> includes the far wings, then your derived FWHM will be strongly biased
> (especially since there are many more pixels in the wings). A
> general rule is that the fitting region should be the size of the
> FWHM. Since the FWHM is what you are trying to determine, you
> might have to iterate. (So if using a 5x5 box gives you a FWHM =
> 1.8, then you might want to recompute it using a 3x3 box.
>
> The pkfit.pro procedure does have some advantages: (1) it fits a
> Gaussian convolved with the pixel size rather than just a Gaussian,
> and (2) it gives lower weight to pixels far from the centroid,and (3)
> it iterates to choose the best (circular) fitting radius, among 3, 5,
> and 7 pixels. But it is very old and ugly code (circa 1988), and
> does a lot of extraneous calculations since it is part of a larger
> fitting package.
>
> Finally, note that if you are only using the FWHM as input to a source
> detection algorithm, then it does not need to be very accurate. --
> Wayne
Of course, another issue is what if your PSF doesn't even remotely
look Gaussian, even within the FWHM. The nice thing about the FWHM is
that it's pretty well-defined for any declining profile... so you
could do something like this (UNTESTED):
; we want to find FWHM of image within a box of size "width" around
"x0", "y0":
pixelvalues = image[x0-0.5*width:x0+0.5*width,
y0-0.5*width,y0+0.5*width]
nbox = n_elements(pixelvalues)
pixelcoords = array_indices(pixelvalues, lindgen(nbox)) + rebin
([x0,y0],2,nbox)-0.5*width
pixelradii2 = total(pixelcoords^2, 1)
; get a smoothed version of pixelvalues to get the average profile.
use a boxcar
; of width 5 as a wild guess. this part of the code could be a lot
smarter.
sortedradii = sort(pixelradii2)
smoothed_pixelvalues = smooth(pixelvalues[sortedradii], 5)
; find the half-max point
maxvalue = max(pixelvalues)
halfmaxpoint = sqrt(interpol(pixelradii2, smoothed_pixelvalues,
0.5*maxvalue))
fwhm = 2. * halfmaxpoint
-Jeremy.
|
|
|
|
| Re: Is there an automated way to estimated FWHM on 2-D image [message #68084 is a reply to message #68075] |
Sun, 20 September 2009 06:12 |
wlandsman
Messages: 743
Registered: June 2000
|
Senior Member |
|
|
On Sep 20, 8:10 am, Jeremy Bailin <astroco...@gmail.com> wrote:
> On Sep 19, 9:02 pm, John Shaw <jds...@udel.edu> wrote:
>
>
>> I was wondering if anyone had a routine for estimating the full-width-
>> at-half-maximum (FWHM) of possible point sources in a 2-D array. Most
>> of the routines I have found and examined request the FWHM for a
>> guassian to be convolved to find the sources.
>
> Would PKFIT in the IDL astronomy library give you what you need? You
> can get the Gaussian dispersion out, which is easy to convert into a
> FWHM.
>
I would just use any Gaussian-2d fitting routine, such as
gauss2dfit.pro in the ITTVIS library, or (preferably) the Gaussian
option of the mp2dfitfun.pro function in Craig Markwardt's fitting
library ( http://www.physics.wisc.edu/~craigm/idl/fitting.html ).
One thing to be careful of is the choice of the fitting region
size. We don't observe Gaussians in real life, and for example,
star images have very extended wings. If your fitting region
includes the far wings, then your derived FWHM will be strongly biased
(especially since there are many more pixels in the wings). A
general rule is that the fitting region should be the size of the
FWHM. Since the FWHM is what you are trying to determine, you
might have to iterate. (So if using a 5x5 box gives you a FWHM =
1.8, then you might want to recompute it using a 3x3 box.
The pkfit.pro procedure does have some advantages: (1) it fits a
Gaussian convolved with the pixel size rather than just a Gaussian,
and (2) it gives lower weight to pixels far from the centroid,and (3)
it iterates to choose the best (circular) fitting radius, among 3, 5,
and 7 pixels. But it is very old and ugly code (circa 1988), and
does a lot of extraneous calculations since it is part of a larger
fitting package.
Finally, note that if you are only using the FWHM as input to a source
detection algorithm, then it does not need to be very accurate. --
Wayne
|
|
|
|
| Re: Is there an automated way to estimated FWHM on 2-D image [message #68087 is a reply to message #68084] |
Sun, 20 September 2009 05:10 |
Jeremy Bailin
Messages: 618
Registered: April 2008
|
Senior Member |
|
|
On Sep 19, 9:02 pm, John Shaw <jds...@udel.edu> wrote:
> Hi,
>
> Longtime reader, first time question:
>
> I was wondering if anyone had a routine for estimating the full-width-
> at-half-maximum (FWHM) of possible point sources in a 2-D array. Most
> of the routines I have found and examined request the FWHM for a
> guassian to be convolved to find the sources.
>
> What I'd like to do, is provide the source positions and get back the
> FWHM.
>
> I.e., I have many images and need to determine the s/n (signal-to-
> noise) and FWHM without interactively working out the values for each
> image (or point source). I can easily determine the background value
> and the flux values for each point, if I provide the FWHM initially,
> but since the FWHM can change significantly from image to image, this
> is a problem.
>
> My overall goal is to get accurate positions of the point sources with
> accuracy estimates (i.e. (s/n) / (FWHM) ). My sticking point seems to
> be the FWHM.
>
> Any help would be appreciated.
>
> Thanks - John
Would PKFIT in the IDL astronomy library give you what you need? You
can get the Gaussian dispersion out, which is easy to convert into a
FWHM.
-Jeremy.
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
